If a provider suspects an invalid vaccination, including those from persons vaccinated outside the U.S., one of two approaches can be taken. Repeating the vaccinations is an acceptable option. Doing so is generally safe and avoids the need to obtain and interpret serologic tests. If avoiding unnecessary injections is desired, judicious use of serologic testing might be helpful in determining which immunizations are needed. This may be particularly helpful in determining tetanus and diphtheria antitoxin levels for children whose records indicate 3 or more doses of DTP or DTaP. This issue is discussed in detail in the ACIP’s “General Best Practice Guidelines for Immunization” chapter titled Special Situations, and summarized in Table 9-1, available at www.cdc.gov/vaccines/hcp/acip-recs/general-recs/special-situations.html.